D. Tierney

Fermi/GBM observations of the ultra-long GRB 091024 - A burst with an optical flash

Aims. In this paper we examine gamma-ray and optical data of GRB 091024, a gamma-ray burst (GRB) with an extremely long duration of T90 ≈ 1020 s, as observed with the Fermi Gamma-ray Burst Monitor (GBM). Methods. We present spectral analysis of all three distinct emission episodes using data from Fermi/GBM. Because of the long nature of this event, many ground-based optical telescopes slewed to its location within a few minutes and thus were able to observe the GRB during its active period. We compare the optical and gamma-ray light curves. Furthermore, we estimate a lower limit on the bulk Lorentz factor from the variability and spectrum of the GBM light curve and compare it with that obtained from the peak time of the forward shock of the optical afterglow. Results. From the spectral analysis we note that, despite its unusually long duration, this burst is similar to other long GRBs, i.e. there is spectral evolution (both the peak energy and the spectral index vary with time) and spectral lags are measured. We find that the optical light curve is highly anti-correlated to the prompt gamma-ray emission, with the optical emission reaching the maximum during an epoch of quiescence in the prompt emission. We interpret this behavior as the reverse shock (optical flash), expected in the internal-external shock model of GRB emission but observed only in a handful of GRBs so far. The lower limit on the initial Lorentz factor deduced from the variability time scale (Γmin = 195 +90 −110) is consistent within the error to the one obtained using the peak time of the forward shock (Γ0 = 120) and is also consistent with Lorentz factors of other long GRBs.

Rest-frame properties of 32 gamma-ray bursts observed by the Fermi Gamma-ray Burst Monitor

Aims: In this paper we study the main spectral and temporal properties of gamma-ray bursts (GRBs) observed by Fermi/GBM. We investigate these key properties of GRBs in the rest-frame of the progeni ...

Localization of Gamma-Ray Bursts Using the Fermi Gamma-Ray Burst Monitor

The Fermi Gamma-ray Burst Monitor (GBM) has detected over 1400 gamma-ray bursts (GRBs) since it began science operations in 2008 July. We use a subset of over 300 GRBs localized by instruments such as Swift, the Fermi Large Area Telescope, INTEGRAL, and MAXI, or through triangulations from the InterPlanetary Network, to analyze the accuracy of GBM GRB localizations. We find that the reported statistical uncertainties on GBM localizations, which can be as small as 1°, underestimate the distance of the GBM positions to the true GRB locations and we attribute this to systematic uncertainties. The distribution of systematic uncertainties is well represented (68% confidence level) by a 3.°7 Gaussian with a non-Gaussian tail that contains about 10% of GBM-detected GRBs and extends to approximately 14°. A more complex model suggests that there is a dependence of the systematic uncertainty on the position of the GRB in spacecraft coordinates, with GRBs in the quadrants on the Y axis better localized than those on the X axis.

Compton scattering in terrestrial gamma-ray flashes detected with the Fermi gamma-ray burst monitor

Max-Planck-Institut fur extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany(Received 30 June 2014; published 20 August 2014)Terrestrial gamma-ray flashes (TGFs) are short intense flashes of gamma rays associated with lightningactivity in thunderstorms. Using Monte Carlo simulations of the relativistic runaway electron avalanche(RREA) process, theoretical predictions for the temporal and spectral evolution of TGFs are compared toobservations made with the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray SpaceTelescope. Assuming a single source altitude of 15 km, a comparison of simulations to data is performedfor a range of empirically chosen source electron variation time scales. The data exhibit a clear softeningwith increased source distance, in qualitativeagreement with theoretical predictions. The simulated spectrafollow this trend in the data, but tend to underestimate the observed hardness. Such a discrepancy mayimply that the basic RREA model is not sufficient. Alternatively, a TGF beam that is tilted with respect tothe zenith could produce an evolution with source distance that is compatiblewith the data. Based on theseresults, we propose that the source electron distributions of TGFs observed by GBM vary on time scales ofat least tens of microseconds, with an upper limit of ∼100 μs.

Anomalies in low-energy gamma-ray burst spectra with the Fermi Gamma-ray Burst Monitor

Context. A Band function has become the standard spectral function used to describe the prompt emission spectra of gamma-ray bursts (GRBs). However, deviations from this function have previously been observed in GRBs detected by BATSE and in individual GRBs from the Fermi era. Aims. We present a systematic and rigorous search for spectral deviations from a Band function at low energies in a sample of the first two years of high fluence, long bursts detected by the Fermi Gamma-ray Burst Monitor (GBM). The sample contains 45 bursts with a fluence greater than 2 × 10 -5 erg/cm 2 (10−1000 keV). Methods. An extrapolated fit method is used to search for low-energy spectral anomalies, whereby a Band function is fit above a variable low-energy threshold and then the best fit function is extrapolated to lower energy data. Deviations are quantified by examining residuals derived from the extrapolated function and the data and their significance is determined via comprehensive simulations which account for the instrument response. This method was employed for both time-integrated burst spectra and time-resolved bins defined by a signal-to-noise ratio of 25 σ and 50 σ . Results. Significant deviations are evident in 3 bursts (GRB 081215A, GRB 090424 and GRB 090902B) in the time-integrated sample (~7%) and 5 bursts (GRB 090323, GRB 090424, GRB 090820, GRB 090902B and GRB 090926A) in the time-resolved sample (~11%). Conclusions. The advantage of the systematic, blind search analysis is that it can demonstrate the requirement for an additional spectral component without any prior knowledge of the nature of that extra component. Deviations are found in a large fraction of high fluence GRBs; fainter GRBs may not have sufficient statistics for deviations to be found using this method.

Pulse properties of terrestrial gamma-ray flashes detected by the Fermi Gamma-Ray Burst Monitor

The Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope has triggered on over 300 terrestrial gamma-ray flashes (TGFs) since its launch in June 2008. With 14 detectors, GBM collects on average ∼100 counts per triggered TGF, enabling unprecedented studies of the time profiles of TGFs. Here we present the first rigorous analysis of the temporal properties of a large sample of TGFs (278), including the distributions of the rise and fall times of the individual pulses and their durations. A variety of time profiles are observed with 19% of TGFs having multiple pulses separated in time and 31 clear cases of partially overlapping pulses. The effect of instrumental dead time and pulse pileup on the temporal properties are also presented. As the observed gamma ray pulse structure is representative of the electron flux at the source, TGF pulse parameters are critical to distinguish between relativistic feedback discharge and lightning leader models. We show that at least 67% of TGFs at satellite altitudes are significantly asymmetric. For the asymmetric pulses, the rise times are almost always shorter than the fall times. Those which are not are consistent with statistical fluctuations. The median rise time for asymmetric pulses is ∼3 times shorter than for symmetric pulses while their fall times are comparable. The asymmetric shapes observed are consistent with the relativistic feedback discharge model when Compton scattering of photons between the source and Fermi is included, and instrumental effects are taken into account.

GBM long and short GRB lightcurve decomposition analysis

The Gamma‐ray Burst duration distribution is bi‐modal with a minimum around 2 s suggesting that there are two separate populations. The GRB time profiles, believed to be the result of internal shocks, reflect the activity of the central engine. A systematic study of the GRB temporal profiles was carried out with a view to understanding the possible differences in the properties of the central engine in the two populations of GRBs. We analyse the temporal profiles of gamma‐ray bursts detected by the Gamma‐ray Burst Monitor onboard the Fermi Gamma‐ray Space Telescope. We deconvolve these profiles into pulses superposed on a quadratic background. The basic pulse shapes are fitted to lognormal functions. It is found that the distributions of rise as well as decay times, their full width at half maximum and the time intervals between successive pulses for long and short GRBs are distinct and each follows a lognormal distribution. We identify several differences in the temporal properties of long and short burs...

Energy‐dependent spectral lags of short GRBs detected by Fermi‐GBM

The Gamma‐ray Burst Monitor (GBM) on board the Fermi satellite has detected over 600 GRBs from its launch in June 2008 up to the end of 2010. Approximately 20% of these are short bursts based on the traditional classification of T90 durations less than 2 seconds. Many long‐duration GRBs exhibit a spectral lag which is seen as high‐energy γ‐ray emission arriving earlier than photons in a low‐energy γ‐ray band. While long‐duration GRBs may have lags ranging from zero to several tens of seconds, short GRBs exhibit negligible or zero lags between energy channels within the 25–1000 keV range of the BATSE Large Area Detectors. The spectral lag has therefore been proposed as a tool to assist in differentiating between long and short duration GRBs. Here we present the spectral lags of the sample of short GRBs detected by GBM in its first 2 years of operation, utilising data from the 12 NaI detectors and 2 BGO detectors on board. The wide energy coverage of GBM (8 keV–40 MeV) allows the dependence of the spectral ...